The use of radio telephones in mobile applications as a convenience or business tool is becoming increasingly popular and has created a demand for reliable instruments compatible with conventional land line telephone systems and the dialing codes used therein. This demand has been met by numerous systems adapted to decode dial codes. However, these systems become increasingly complex as the number of digits in the codes increases to meet the increased user demand. This has resulted in the development of radio telephone decoders which are complex, costly, difficult to maintain, difficult to install and tailor to a specific application and require special mounting and installation considerations.
For instance, U.S. Pat. No. 3,859,475 on "Decoder Channel Selector And Enunciator System In An Airborne Radio Telephone System" issued to P. Wulfsberg et al on Jan. 3,1975 teaches the concept of a radio telephone decoder of the type which determines a series code based upon the transition of a tone signal between 600 and 1500 Hz but it is limited in its ability to cope with the ever increasing number of radio telephones and is not compatible with a standard seven digit dial code due to the five digit selector limitation in the circuitry. This system, while an improvement over the bulky relay decoders used in prior art does not appear to provide an adaptable, field strapable decoding system or sub-miniature structure which would simplify installation and maintenance.